Aspects of Solid-state Polycondensation

According to the principles of polycondensation, all of the above reactions will also take place during SSP. The conditions for the latter, however, are different as this process is carried out at lower temperatures in a non-homogeneous environment. In order to examine the kinetics of SSP, some assumptions have to be made to simplify the analysis. These are based on the idea that the reactive end groups and the catalyst are located in the amorphous regions. Polycondensations in the solid state are equilibrium reactions but are complicated by the two-phase character of the semicrystalline polymer. 

During the studies carried out on this process some unusual behavior has been observed. Such results have led some authors to the conclusion that SSP is a diffusion-controlled reaction. Despite this fact, the kinetics of SSP also depend on catalyst, temperature and time. In the later stages of polymerization, and particularly in the case of large particle sizes, diffusion becomes dominant, with the result that the removal of reaction products such as EG, water and acetaldehyde is controlled by the physics of mass transport in the solid state. This transport process is itself dependent on particle size, density, crystal structure, surface conditions and desorption of the reaction products. 

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Flory outlined [3] that the definition of polycondensation is necessarily based on kinetic aspects and not on the structure of polycondensates, because numerous polycondensates can also be prepared by ROP which usually proceeds as chain-growth polymerization. Flory s definition of step-growth polymerization is limited to polycondensations and polyadditions in the melt or in solution, and does not include solid-state polycondensations. Hory s definition of step-growth polymerizations is based on point 1. 

Continuous processes are currently used in the manufacture of PET. Several models have been developed, with the aim of contributing to a better design and operation. A brief discussion of their main assumptions and predictive capacities follows. Since polycondensation in solid-state and in film-forming devices has been analyzed previously, only the specific aspects of the initial process stages still needs to be covered. 

The deep insight in the structure of the chains is often accompanied by little attention to their association in the solid state on the other hand, some authors are only concerned by the material without paying great attention to the macromolecule itself. However, some articles associate these two aspects, as did Purukawa et al. [117], who prepared polysiloxane-6-polyamide by solution polycondensation (xylene/NMP) of 3,3, 4,4 -diphenylsulfonetetracarboxylic dianhydride, 2,2-bis[4-(4-aminophenoxy)phenyl]propane, and Qf,w-diammo-poly-siloxane (the synthesis belongs to the techniques listed in Section 2.3). The chains were analyzed by H, and Si NMR, as well as by infrared spectroscopy, whereas the spin-lattice relaxation time was measured by solid-state NMR. A kinetic study of the imidization was carried out in NMP solution it showed that the value of the activation energy is partly determined by the solvation of the amide groups and polyamic acids. 

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